A Cu—Sn—Pb based alloy for casting is standardized in JISH2207. A Cu—Sn—Pb based sintered alloy having the same composition as JISH2207 is bonded to a backing metal. The resultant bimetal bearing is used for a bush of an automatic transmission, such as a sun gear, an oil pump, a transfer, an end bearing, and a pinion. Pb is dispersed to form a low-melting secondary-phase in such copper alloy and behaves as a lubricating component. Pb also reduces hardness of copper-alloy as a whole and improves conformability. As a result, the seizure resistance of the copper alloy is improved.
Patent Document 1: Japanese Unexamined Patent Publication (kokai) No. 2001-107106 proposes a bimetal bearing of sintered copper-alloy. This copper alloy contains 0.4% or less P and 12% or less Sn by weight percentage as essential elements, and further contains 10% or less Ni, 5% or less Ag, 5% or less Pb and 5% or less Bi as optional components. Mo and/or W are dispersed in the sintered copper alloy as hard particles. Patent Document 1 describes that Bi and Pb improve conformability; and, Ag enhances corrosion resistance and strength. This effect is realized as long as Ag is dissolved in solid copper (Cu).
Patent Document 2: Japanese Unexamined Patent Publication (kokai) No. 2005-350722 relates to a Pb-free Cu—Bi-hard-particle-based sintered alloy and proposes to refine the Bi-phase particles by means of the hard-particles so as to achieve sliding properties equivalent to the Pb-containing copper alloy.
Patent Document 3: Japanese Patent No. 3560723 (Japanese Patent Application Hei 8-57874) proposes a plain-bearing copper-alloy which is Pb free. Ag, Sn, Sb, In, Mn, Fe, Bi, Zn, Ni and/or Cr (except for combination of only Ag and Sn) is dissolved in the solid Cu matrix. In particular, Ag is solid-dissolved in a non-equilibrium state. No secondary phase of these elements is essentially formed. A soft secondary phase, such as Pb, is absent in the copper alloy, and therefore such alloy is called Pb free. According to the description of Patent Document 3, the specified solute elements, for example, Ag and Bi, concentrate on the surface of copper alloy during sliding and form an excellent seizure-resistant material. The properties of the copper alloy are equivalent to those of a Pb-containing copper alloy.
From a binary Cu—Bi phase diagram, solid Cu and solid Bi have essentially no solubility with each other. Cu and Bi are mutually dissolved in a liquid phase and form two separate solid phases. Therefore, in order to dissolve Bi in the solid copper-alloy, Cu and Bi in molten state must be extremely rapidly cooled to produce a melt-quenched Cu—Bi powder. However, the cooling rate is higher than an industrially possible one.    Patent Document 1: Japanese Unexamined Patent Publication (kokai) No. 2001-107106    Patent Document 2: Japanese Unexamined Patent Publication (kokai) No. 2005-350722    Patent Document 3: Japanese Patent No. 3560723